Whims of an Accreting Young Brown Dwarf: Exploring Emission Line Variability of 2MASSW J1207334-393254

Abstract

We report the first comprehensive study of emission line variability in an accreting young brown dwarf. We have collected 14 high-resolution optical spectra of 2MASSW J1207334-393254 (M8), a likely member of the nearby 8-million-year-old TW Hydrae association. These spectra show a variety of emission lines that are commonly seen in classical T Tauri stars. Halpha line in particular shows dramatic changes in shape and intensity in our dataset, both on timescales of several weeks and several hours. In spectra from late-January, the line is relatively weak and only slightly asymmetric. Spectra from mid- and late-March show intense, broad and asymmetric Halpha emission, indicative of on-going disk accretion. We estimate that the accretion rate could have changed by a factor of 5-10 over ~6 weeks in this brown dwarf. March spectra also reveal significant `quasi-periodic' changes in the Halpha line profile over the course of a night, from clearly double-peaked to nearly symmetric. These nightly profile changes, roughly consistent with the brown dwarf's rotation period, could be the result of a redshifted absorption feature coming into and out of our line of sight; when the profile is double-peaked we may be looking into an accretion column, flowing from the inner disk edge on to the central object, indicating that the accretion is probably channelled along the magnetic field lines. Our findings provide strong support for the magnetospheric accretion scenario, and thus for the existence of large-scale magnetic fields, in the sub-stellar regime (abridged).